In the field of battery powered consumer applications, there is a continual drive to reduce the cost and power consumption of such applications. Reducing the die size of integrated circuit devices used within such battery powered consumer applications is one important technique for reducing costs.
Many battery powered consumer applications require clock generator circuits to generate timing reference signals within a mixed signal integrated circuit device. For example an accelerometer integrated circuit device may comprise a MEMS (micro-electro-mechanical system) sensor for sensing acceleration, and a mixed signal integrated circuit for transforming the mechanical acceleration into an electrical signal. The electrical image of acceleration is measured through an ADC (analogue to digital converter), followed by digital data processing. The processed data is then provided to a processor.
The time frame for measuring and converting the acceleration is fixed (e.g. 100 us), while the data rate may be variable (e.g. 0.1 Hz to 1 kHz). During the 100 us measure and convert time frame, a ‘high frequency’ timing reference signal (e.g. 1 MHz) is required to enable the acceleration to be measured, converted (from analogue to digital) and processed within the time frame. This high frequency timing signal is only enabled during the measure and convert time frame. A second ‘low frequency’ reference timing signal (e.g. 32 kHz), which is ‘always on’ and when the high frequency timing signal is not enabled. In this manner, the high frequency timing signal is only used when required, with the integrated circuit components being switched to the low frequency reference timing signal at other times to reduce power consumption.
Within such a mixed signal integrated circuit device, the handover between two reference timing signals must be performed in an expeditious manner to minimize any delay which could impact the overall time scaling. Clock generators are typically realised through oscillator circuits. There are various different types of oscillator circuits. Relaxation oscillators have a short start-up (enabling) time but they are usually larger in terms of die size and consume more current than, say, ring oscillators. Furthermore, relaxation oscillators potentially generate kickback noise due to the fact that they compare a ramp with a reference signal. This comparison is done with a comparator (not needed in ring oscillators) whose inputs are the reference and the ramp. Once the ramp reaches the reference the comparator changes state, which results in changes around the ramp node. This change can be coupled back to the reference node through parasitics around the comparator input, and thus can perturb the reference. Usually the reference is shared with other circuits who could suffer due to this kick-back noise. Accordingly, despite their short start-up (enabling) time, relaxation oscillators are not suitable for use within many integrated circuit devices.
Ring oscillators do not suffer from the potential to generate kickback noise in the same way as relaxation oscillators, and are usually smaller in terms of die size and consume less current than relaxation oscillators. As such, ring oscillators are generally the preferred type of ring oscillator for use within many integrated circuit devices. However, ring oscillators have the drawback that they can suffer from large start-up (enable) times, requiring earlier start-up of the ring oscillators in order to ensure they are ready for handover in time. The need to start-up the ring oscillators early reduces the effectiveness of using different frequency reference timing signals within a mixed signal integrated circuit to reduce power consumption.